There are known in the art progressive lenses which provide a smooth transition of optical power between a near viewing region and a far viewing region.
Winthrop U.S. Pat. Nos. 4,861,153 and 5,123,725 disclose progressive lenses which are constructed in accordance with the bipolar principle. As is explained in these patents, in such a lens both the near vision zone and the distance vision zone are reduced to mathematical points or poles.
Further as is known in the art, lenses are constructed in accordance with a power law, which determines a portion of the lens meridian, which in turn may, as here, lie entirely in the Y-Z plane of a Cartesian coordinate system. Generally, lenses manufactured in accordance with the disclosures of the Winthrop patents have power laws the curves of which have relatively low slopes or gradients adjacent to the poles and a higher slope or gradient intermediate the poles. In accordance therewith, the '153 patent discloses a first numerical example of an ordinary progressive lens which is constructed in such a way as to provide relatively large areas of optical stability at the poles while minimizing astigmatism. This patent discloses a second numerical example which utilizes a linear power law to provide what is said to be possibly the lowest level of astigmatism possible in a progressive lens with umbilic vertical meridian.
The '725 patent adds third and fourth numerical examples, both of which are constructed in accordance with a power law of the same type as that of the first example of the '153 patent. The third example relates to what is identified as an occupational lens which has improved near utility at the expense of distance utility. While the fourth example, which is said to be a dynamic activity lens, has improved distance utility at the expense of near utility.